Photographic silver halide emulsions are dispersions of radiation sensitive silver halide microcrystals, referred to as grains, capable of forming a latent image. Photographic silver halides exclude silver fluoride, which is water soluble, and silver iodide, which, though highly useful in minor proportions, as a major grain component does not efficiently form developable latent images. Although photographic silver halide emulsions prepared by single jet precipitation techniques have been long known to contain some tabular grains, the photographic advantages offered by the presence of tabular grains in silver halide emulsions was not appreciated until relatively recently.
Depending upon the intended photographic application and the halide content of the tabular grains, tabular grain emulsions have been recently disclosed in which tabular grains of (i) 0.5 micrometer (hereinafter designated .mu.m) or less in thickness, more typically 0.3 .mu.m or less in thickness, and optimally less than 0.2 .mu.m in thickness (ii) having an average aspect ratio of at least 5:1, more typically greater than 8:1, and (iii) accounting for greater than 35 percent, more typically greater than 50 percent, of the total grain projected area of the emulsion have been disclosed. Disclosed advantages have included increased speed, improved developability, improved speed-granularity relationships, increased sharpness, increased blue and minus blue speed separations, higher developed silver covering power of fully forehardened emulsions, reduced crossover in dual coated radiographic elements, higher transferred image densities at reduced silver coverages in image transfer photography, and reduced thermal variance and rereversal in direct reversal applications. Illustrative of high and intermediate aspect ratio tabular grain emulsions, their methods of preparation, and their photographic advantages are the following:
(T-1) Wilgus et al U.S. Pat. No. 4,434,226, PA0 (T-2) Kofron et al U.S. Pat. No. 4,439,520, PA0 (T-3) Daubendiek et al U.S. Pat. No. 4,414,310, PA0 (T-4) Abbott et al U.S. Pat. No. 4,425,425, PA0 (T-5) Wey U.S. Pat. No. 4,399,215, PA0 (T-6) Solberg et al U.S. Pat. No. 4,433,048, PA0 (T-7) Dickerson U.S. Pat. No. 4,414,304, PA0 (T-8) Mignot U.S. Pat. No. 4,386,156, PA0 (T-9) Jones et al U.S. Pat. No. 4,478,929, PA0 (T-10) Evans et al U.S. Pat. No. 4,504,570, PA0 (T-11) Maskasky U.S. Pat. No. 4,400,463, PA0 (T-12) Wey et al U.S. Pat. No. 4,414,306, PA0 (T-13) Maskasky U.S. Pat. No. 4,435,501, PA0 (T-14) Abbott et al U.S. Pat. No. 4,425,426, PA0 (T-15) Research Disclosure, Vol. 232, Aug. 1983, Item 23212, and PA0 (T-16) Research Disclosure, Vol. 225, Jan. 1983, Item 22534.
Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
While initial investigations of tabular grain emulsions focused on serving predominantly higher speed photographic applications, more recently attention has been focused on relatively slower speed emulsions.
Daubendiek et al U.S. Ser. Nos. 790,692 and 790,693, both filed Oct. 23, 1985, refiled Aug. 1, 1986, as U.S. Ser. Nos. 891,803 and 891,804, respectively, all commonly assigned, disclose the utility of small, thin tabular grain emulsions in color photograpay. Specifically, the utility is disclosed in blue and minus blue recording layers of color photographic elements of emulsions having tabular grain mean diameters in the range of from 0.2 to 0.55 .mu.m, wherein the grains have average aspect ratios greater than 8:1 and account for greater than 50 percent of the total grain projected areas.
A unifying theme running through these various tabular grain emulsion disclosures is the importance of having the tabular grains account for a high proportion of the total grain projected area, where the term "projected area" is used in the same sense as the terms "projection area" and "projective area" commonly employed in the art; see, for example, James and Higgins, Fundamentals of Photographic Theory, Morgan and Morgan, New York, p. 15. These disclosures also emphasize the importance of increasing average aspect ratios, where aspect ratio is defined as the ratio of the diameter of a tabular grain to its thickness. The diameter of a tabular grain is the diameter of a circle whose area is equal to the projected area of the tabular grain. It is generally recognized and accepted that to the extent (i) the average aspect ratio of a tabular grains and (ii) the percentage of the total grain projected area accounted for by tabular grains, can be increased, the photographic properties of the tabular grain emulsions can be improved.
All photographically useful silver halides form grains--i.e., microcrystals--of a cubic crystal lattice structure. The silver halide grains are bounded by cubic or {100} crystallographic planes, octahedral or {111} crystallographic planes, and/or rhombic dodecahedral or {110} crystallographic planes, the latter occurring only rarely. {100} (occasionally also referred to as {200}), {111}, and {110} are Miller index assignments of the grain crystal faces. Regular grains bounded entirely by {100} crystal faces form regular cubes, regular grains bounded by {111} crystal faces form regular octahedra, and regular grains bounded by {110} crystal faces form regular rhombododecahedra.
It has been recently observed that there are four additional families of crystallographic planes that can bound cubic crystal lattice silver halide grains:
(1) Maskasky U.S. Ser. No. 771,861, titled SILVER HALIDE PHOTOGRAPHIC EMULSIONS WITH NOVEL GRAIN FACES (1), discloses emulsions containing silver halide grains bounded by hexoctahedral crystallographic planes. Hexoctahedral crystallographic planes satisfy the Miller index assignment {hkl}, wherein h, k, and l are integers greater than zero, h is greater than k, and k is greater than l. Most commonly h is 5 or less.
(2) Maskasky U.S. Ser. No. 772,228, titled SILVER HALIDE PHOTOGRAPHIC EMULSIONS WITH NOVEL GRAIN FACES (2), discloses emulsions containing silver halide grains bounded by tetrahexahedral crystallographic planes. Tetrahexahedral crystallographic planes satisfy the Miller index assignment {hh0}, wherein 0 is zero, h and k are integers greater than 0 and different from each other. Most commonly h and k are no greater than 5.
(3) Maskasky U.S. Ser. No. 772,229, titled SILVER HALIDE PHOTOGRAPHIC EMULSIONS WITH NOVEL GRAIN FACES (3), discloses emulsions containing silver halide grains bounded by trisoctahedral crystallographic planes. Trisoctahedral crystallographic planes satisfy the Miller index assignment {hhl}, wherein h and l are integers greater than zero and h is greater than l. Most commonly h is no greater than 5.
(4) Maskasky U.S. Ser. No. 772,230, titled SILVER HALIDE PHOTOGRAPHIC EMULSIONS WITH NOVEL GRAIN FACES (4), discloses emulsions containing silver halide grains bounded by icositetrahedral crystallographic planes. Icositetrahedral crystallograpaic planes satisfy the Miller index assignment {hll}, wherein h and l are integers greater than zero and h is greater than l. Most commonly h is no greater than 5.
These patent applications were all filed Sept. 3, 1985, and refiled as U.S. Ser. Nos. 881,768, 881,769, 882,112, and 882,113, on July 3, 1986, and are all commonly assigned. The novel crystallographic faces were made possible by finding grain growth modifiers capable of reducing the rate of growth of the crystal face desired, since it is the slowest growing crystal faces that bound the grains and give them their surfaces.
(5) Maskasky U.S. Ser. No. 772,271, filed Sept. 3, 1985, commonly assigned, titled SILVER HALIDE PHOTOGRAPHIC EMULSIONS WITH NOVEL GRAIN FACES (5) discloses tabular grain emulsions having opposed major octahedral or 55 111} faces which are ruffled by the deposition of silver halide thereon. By the use of grain growth modifiers ruffling deposits capable of forming any of the remaining six families of crystallographic planes possible with cubic crystal lattice silver halide grains can be formed.